For centuries, windmills have been used for pumping water and grinding grain. More recently, so called wind turbines have been used to generate electric power from intercepted wind power. A basic problem in harnessing power from wind is the low density of power per unit volume handle. The maximum amount of power that may be extracted from the wind is proportional to the circular cross-sectional area through which the blades rotate, multiplied by the square of wind velocity. More specifically, the extractable power is the total kinetic energy of the air moving through the turbine's cross-section per unit time. Of this available power, however, the actual turbine will only extract a fraction, thereby defining the turbine's efficiency.